1. Field of the Invention
The present invention relates to an optical distance measuring device which illuminates an object with a pulsed light beam, receives part of a retroreflection light beam reflected and returned from the object, and measures the delay time to detect the distance to the object and the direction thereof, and more particularly to a device which is to be mounted on an automobile to monitor the periphery of the automobile, and which is to be applied to an obstacle warning device or a cruise controlling device for a vehicle.
2. Description of the Related Art
Conventionally, a device which measures the time period between emission of a pulsed light beam and reception of a light beam reflected from an object, to determine the distance to the object has been used in various fields. Among optical distance measuring devices of this kind, a periphery monitoring device which is to be mounted on a vehicle is used in a larger number. Such a device is used mainly in a vehicle gap controlling device, as a sensor which measures the distance to a preceding vehicle.
An example of a conventional device which measures a distance by such a method will be described with reference to JP-A-8-122437. FIG. 10 is a diagram of the conventional art example. The reference numeral 10 denotes light transmitting means which is configured by a light emitting element 11, a light emission driver 12, an illumination lens 13, and scanning means 14 for scan-illuminating a transmitted light beam in a predetermined angular range. The reference numeral 20 denotes light receiving means for receiving a reflected pulsed light beam which is reflected and returned from an object. The light receiving means is configured by a photoelectric converting element 21, an amplifier 22 which converts a photocurrent into a voltage, and a converging lens 23 for receiving light. The amplifier 22 is configured by an STC (Sensitivity Time Control) circuit 22c and a variable gain amplifier 22d. The reference numeral 30c denotes retroreflection time detecting means for detecting a retroreflection time of a reflected pulsed light beam which is reflected by the object and received by the light receiving means 20. The retroreflection time detecting means is configured by comparing means 34c for comparing an output S20 of the light receiving means 20 with a predetermined value VO, a peak hold circuit 35c which detects the peak value of the light reception signal S20, and a time measurement circuit 33c. The reference numeral 40b denotes a calculation unit which controls the illumination direction and timing of the transmitted light beam, and calculates the distance to the object, from an output of the light reception time detecting means 30c. 
Next, the operation of the device will be described with reference to FIG. 11. The light transmitting means 10 illuminates a transmitted light beam in a predetermined direction on the basis of a signal from the calculation unit 40b. FIG. 11 shows a case where it is assumed that an object QA is a vehicle which is running in front of the device, and an object QB is an article such as a signboard above a road. The light receiving means detects reflected pulsed light beams which are reflected by the objects QA and QB and outputs a reflected pulsed light beam S20A from the object QA, and a reflected pulsed light beam S20B from the object QB as shown in FIG. 11. The output S20 of the light receiving means 20 is supplied to the light reception time detecting means 30c, and then compared with the predetermined value by the comparing means 34c to supply a signal indicating that the output S20 is larger than the predetermined value VO, to the time measurement circuit 33c. The time measurement circuit 33c uses a transmission light signal ST output from the calculation unit 40b, as a start signal, and an output of the comparing means 34c as a stop signal. Namely, the circuit measures a time difference between illumination and reception of the transmitted light beam. As shown in FIG. 11, in the case where reflected pulsed light beams respectively from two objects are detected, the comparing means 34c produces two stop signals PA and PB. The time measurement circuit measures time periods ta and tb (indicated in the figure). Each of the time periods ta and tb is a time period when a light beam is reflected and returned from an object. The distance to an object can be calculated from such a time period and the velocity of light by the following expression:
La=xc2xd*(velocity of light)*ta.
As described above, the conventional art example discloses that timings when reflected pulsed light beams due to objects exceed a predetermined value are detected, so that the distances to the objects can be detected. When also a distant object of a low reflectance, such as a dirty preceding vehicle, a vehicle without a reflector, or a laterally-directed vehicle can be detected, it is possible to further enhance safety. In order to attain this, a sensitive photoelectric converting element or an optical system of higher sensitivity may be used. In this case, however, a situation where a plurality of reflected pulsed light beams are detected with respect to one transmitted pulsed light beam often occurs, and troubles such as described below are caused to reduce safety. Specific examples will be described with reference to FIGS. 12A to 12D.
FIGS. 12A to 12D show cases where a destination signboard or the like exists in a vertical angular range of the transmitted light beam. Even in the case where the signboard is relatively small, when the sensitivity of the element is enhanced as described above, the device receives not only a reflected pulsed light beam from a vehicle but also that from the signboard. FIG. 12A shows a case where the object vehicle QA to be detected is in front of the signboard QB. In the light reception signal S20, the reflected pulsed light beam S20A from the object vehicle, and the reflected pulsed light beam S20B from the signboard QB are separated from each other. Therefore, the retroreflection times ta and tb of the light beams can be detected so that the distances can be measured. FIG. 12B shows a state where the object vehicle QA and the signboard QB are close to each other and hence the reflected pulsed light beams from the objects overlap with each other. The occurrence of this overlapping depends on the pulse width of the transmitted pulsed light beam. For example, it is assumed that the transmitted pulsed light beam has a pulse width of 50 nS. When the relative distance between the objects QA and QB is not longer than 7.5 m, overlapping occurs. In such a state where a plurality of reflected pulsed light beams overlap with each other, the conventional art example can detect only the first reflected pulsed light beam. Therefore, only the retroreflection time ta corresponding to the reflected pulsed light beam from the object vehicle QA can be detected, and the detection of the signboard QB is disabled. As a result, only the distance to the object vehicle QA is output. FIG. 12C shows a state where the preceding vehicle QA is remoter than the signboard QB but the two reflected pulsed light beams remain to overlap with each other. In this case also, in the same manner as FIG. 12B, the two reflected pulsed light beams are detected as one reflected pulsed light beam. Therefore, the reflected pulsed light beam from the object vehicle QA is not detected, and only the time tb corresponding to the reflected pulsed light beam from the signboard QB is detected. As a result, only the distance to the signboard is output. FIG. 12D shows a state where the object vehicle QA is advanced to a remoter position, and the reflected pulsed light beam from the signboard QB and that from the object vehicle QA are again separated from each other. Both the distances to the object vehicle QA and the signboard QB are again enabled to be measured. In the states of FIGS. 12B and 12C, therefore, it is impossible to judge from the output of the conventional art example whether the output distance is the distance to the object vehicle QA or that to the signboard QB.
A case where the device of the conventional art example is applied to a vehicle gap control system will be considered. A vehicle gap control system maintains the distance to a preceding vehicle constant. In the system, it is important to correctly detect the distance to a preceding vehicle and the relative velocity. In a state where reflected pulsed light beams from two objects overlap with each other as described above, however, there arises an error in the relative velocity which is calculated from the detected distance. In a scene where the preceding vehicle passes under the signboard, as described above, the possibility that the distance to the signboard is temporarily recognized as that to the preceding vehicle as shown in FIG. 12C is high. Therefore, the preceding vehicle is judged as if it is temporarily decelerated. This erroneous judgement may cause the own vehicle to be unnecessarily controlled to be decelerated. Such a control provides the driver with unpleasant feelings. In the scene of FIG. 12D where the distance to the preceding vehicle is again correctly detected, the preceding vehicle is judged as if it is suddenly accelerated. This erroneous judgement may cause the own vehicle to be accelerated. Similar phenomena occur also in a scene where a reflective member such as a road mark drawn on the road surface, or a wall of an entrance of a tunnel exists above and below the travelling path of the preceding vehicle. As the device is made more sensitive, such a situation is caused more frequently, thereby producing a problem which is nonnegligible in a practical use.
It is an object of the invention to obtain a distance measuring device for a vehicle which, even when one transmitted light beam impinges on a plurality of reflective members, can measure distances respectively to the reflective members, and particularly, even when two objects are close to being within a distance corresponding to the pulse width of the transmitted light beam and reflected pulsed light beams from the objects overlap with each other, can output correct distances, does not output incorrect distances, or outputs attribute information indicating that two reflected pulsed light beams overlap with each other, with being added to distance data, thereby enhancing the reliability.
The device of a first aspect of the invention includes: light transmitting means for scan-illuminating a pulsed light beam in an external predetermined angular range; a photoelectric converting element which receives a pulsed light beam that is externally reflected, and which converts the light beam into an electric signal; and light receiving means comprising at least two amplifiers which have different gains, and further includes: a plurality of retroreflection time detecting means for receiving output signals of the amplifiers, and for respectively detecting a retroreflection time(s) of a single or plural reflected pulsed light beams in the output signals; and distance calculating means for, on the basis of outputs of the plural retroreflection time detecting means, calculating distances to objects. With respect to a signal output of a high gain, the plural amplifiers improve the detectability in the case where the power of a light reception signal of a reflected pulsed light beam is low, and, with respect to a signal output of a low gain, improve the property of separating overlapping reflected pulsed light beams. From two light reception signals, the distance calculating means calculates distances to plural objects according to a predetermined procedure.
In the device of a second aspect of the invention, each of the retroreflection time detecting means in the device of the first aspect of the invention detects a timing when the reflected pulsed light beam contained in the input light reception signal from the corresponding amplifier rises, and that when the reflected pulsed light beam falls, and records the elapsed times. Even when the reflected pulsed light beam signal is clipped to a certain level in the amplification by the amplifier, the retroreflection time of a reflected pulsed light beam can be detected from the recorded rising and falling timings.
According to a third aspect of the invention, in the device of the first or second aspect of the invention, the distance calculating means changes a method of calculating distance data on the basis of the outputs of the plural retroreflection time detecting means, in accordance with a first pulse width of a reflected pulsed light beam obtained from one of the amplifiers, the amplifier having a higher gain. In accordance with the state of a received reflected pulsed light beam, an adequate distance calculation is enabled.
In the device of a fourth aspect of the invention, when the first pulse width of the reflected pulsed light beam obtained from the amplifier having a higher gain is larger than a predetermined pulse width, a distance is calculated from an output(s) of retroreflection time detecting means connected to an amplifier(s) having a gain which is lower than the higher gain. According to this configuration, an error which may be produced when plural reflected pulsed light beams are overlappingly received is prevented from being produced in distance measurement data.
In the device of a fifth aspect of the invention, distances are calculated from outputs of the plural retroreflection time detecting means, and attribute information of corresponding one of the distance data is output with being added to the corresponding distance data, the attribute information being set in accordance with the first pulse width. According to this configuration, it is possible to prevent an apparatus which uses distance measurement data output from the device of the invention, from being caused to erroneously operate by incorrect distance measurement data.